Use -fPIE for clang builds

Executables can't be produced without this option using clang available
under Ubuntu 20.04, as linking fails with a lot of errors of the form

/usr/bin/ld: Swig/wrapfunc.o: relocation R_X86_64_32S against `.rodata'
can not be used when making a PIE object; recompile with -fPIE

.github/workflows/ci.yml

@@ -83,6 +83,9 @@ jobs:
           if test '${{ matrix.compiler }}' = 'clang'; then
             CC="clang"
             CXX="clang++"
+
+            CFLAGS="$CFLAGS -fPIE"
+            CXXFLAGS="$CXXFLAGS -fPIE"
           elif test -n "$GCC"; then
             CC="gcc-$GCC"
             CXX="g++-$GCC"